1. Field of the Invention
The present invention relates to a triangle wave generating circuit.
2. Description of the Related Art
A switching power supply that generates a desired direct-current voltage by a PWM (Pulse Width Modulation) control is generally known as a power supply for generating a direct-current voltage used in electronic equipment such as a personal computer. For example, Japanese Patent Application Laid-Open Publication No. 2007-174853 discloses a DC-DC convertor that compares a smoothed output voltage and a triangle wave or a saw-tooth wave by a PWM comparator and controls on and off of a switching device according to an output of the PWM comparator.
As such, a PWM signal may be generated using the triangle wave or the saw-tooth wave as a signal for voltage comparison with the output voltage so that the desired direct-current voltage may be generated.
In such electronic equipment as to incorporate a number of electronic circuits, it is desirable from a view-point of reducing maximum power consumption of the electronic equipment as a whole that a plurality of switching power supplies, which are PWM-controlled with phases different from one another, are incorporated, to disperse loads connected to each of the switching power supplies. As illustrated in FIG. 13 or 14, for example, the PWM control may be performed with phases shifted by 180° from each other by outputting the triangle wave TW1 from a common triangle wave generating circuit 8a or 8b and outputting from an inverting amplifier circuit 9 the triangle wave TW2 obtained by inverting the triangle wave TW1.
As illustrated in B2 and C2 of FIG. 15, for example, however, when the triangle wave TW1 input to the inverting amplifier circuit 9 is not a symmetrical triangle wave, the triangle wave TW2 output from the inverting amplifier circuit 9 does not form a triangle wave with a phase shifted by 180° relative to the phase of the triangle wave TW1, and thus, the PWM control may not be performed with the phases shifted by 180° from each other. As illustrated in B and C of FIG. 16, for example, when the triangle wave TW1 has a high frequency (short period), the triangle wave TW2 has a rounded waveform due to the slew rate, etc., of an operational amplifier 90, and thus, accuracy of the PWM control will be reduced.
For this reason, in the case of using a plurality of switching power supplies that are PWM-controlled with the phases different from one another, asymmetrical triangle waves including the saw-tooth wave may not be used as the signal for the voltage comparison and the switching power supplies may not be increased in switching frequency.